Opam alternative to using pkg-config

some of the libraries provided by nixos do not have a .pc file packaged with their artifacts. nixos’ environment is also a little bit hostile to pkg-config in general (you always have to write a shell.nix definition file to get into a useful dev environment which pkg-config can work on).
is there a known way for opam to cope with this? somehow promising it that conf-* packages are reachable from environment?